Seed crop harvesters, now most commonly used in the form of self-propelled or pull-type combines, incorporate threshers which by and large have not undergone any significant changes for the past hundred years. Briefly, power driven threshers have a rotating cylinder the periphery of which is fitted with axially oriented beater wings, usually provided with peripheral teeth, and a cooperating concave. The concave is trough-shaped and is defined by a multiplicity of axially oriented, parallel bars carried by circumferential supports which are constructed to form relatively large, typically square openings through which loose kernels and an undesirably large amount of chaff gravitationally drop for collection beneath the concave. Threshing is effected by passing the seed crop through a typically adjustable, closely controlled, narrow gap between the beater wings of the cylinder and the rasp bars of the concave. As the seed crop passes through this gap the beater wings and the rasp bars mechanically strip the kernels from the straw.
The bulk of the loose kernels and much chaff drops gravitationally through the openings in the concave. The remaining loose kernels, together with the plant growth or straw, are directed from the concave, usually via a rotatory beater to straw walkers which typically comprise a number of parallel, elongated bars that cyclically move up and down relative to each other to in effect throw the straw into the air. The kernels, which have a greater specific weight than the straw, drop faster than the straw and thus become segregated therefrom so that they can be collected beneath the straw walker.
Since threshers of the type described above mechanically strip the kernels from the straw, the gap or spacing between the rotating cylinder and the concave must be small, otherwise the percentage of the kernels not stripped from the plant and, therefore, not harvested but discharged from the combine with the straw would reach unacceptable levels. Conversely, the gap must necessarily be wider than the size of the kernels, to limit cracking of the kernels. For wheat, for example, a gap width of as little as 1/8 inch and, since the entire, relatively large volume of the crop must be fed through this narrow gap, a cylinder operating at a high relative speed is essential in order to effectively strip the kernels and obtain an adequate throughput so that harvesting of the field can progress at a reasonable speed. This makes it necessary to rotate a standard 22 inch diameter cylinder for threshing wheat, for example, in the range of between 1,000-1,200 rpm, loading to relative surface speeds between the cylinder and the concave of up to 80 mph. This results in very short throughput times for the seed crop, typically in the range of no more than 1/50 to 1/100 sec. or less during which the kernels must be stripped from the plants.
Moreover, it is necessary to evenly distribute the incoming seed crop over the length of the thresher. If, for example, there is a localized bunching of the seed crop in one area of the thresher, the cylinder frequently becomes jammed and threshing comes to a halt. It can only be resumed after a disassembly of the thresher, the removal of the materials jammed therein, and its reassembly, causing down times from 1/2 hour to several hours during which both the combine and workman operating it are idle. Harvesting costs are correspondingly increased, thereby decreasing the overall efficiency and profitability of the combine.
Even with optimal adjustments of the gap width between the rasp bars of the concave and the beater bars of the cylinder an appreciable portion of the kernels are either cracked, not threshed or both and therefore lost. This reduces the overall quality of the harvested kernels (due to the presence of cracked kernels) and/or lowers the yield, both of which adversely affect the efficiency of the harvesters and the profit that can be obtained with them.
A further drawback of conventional threshers resulting from their above summarized construction and operation is that the components of the thresher, especially the cylinder and the concave, are subjected to large forces and to much wear and tear. This requires a relatively massive construction utilizing high strength materials. Further, it is frequently necessary to specially treat the components and/or use special material to reduce their wear and enhance their operating lives. This makes conventional threshers relatively expensive to build and operate.
As a further consequence of the high-speed force feeding of the seed crop through the narrow gap of the thresher such combines require large power plants which consume large amounts of expensive fuel. Much of this power is lost due to friction between the cylinder, the straw and the concave and for stripping, i.e., tearing the kernels from the straw. Harvesting costs are thereby further increased.
In spite of the many shortcomings of conventional threshers, only the most important of which have been mentioned above, they have undergone no significant changes over the last 100 years. They are used today in even the most technologically advanced and efficient combines, thousands of which are in operation throughout the U.S. and worldwide. Yet, the best that can be achieved with such combines is an attempt to strike an optimal balance among two highly undesirable side effects which are necessarily encountered with such threshers, namely kernel cracking and kernel loss due to nonthreshing.